Electron transfer through a lipid-bilayer-membrane-aqueous-solution interface and kinetics of the oxidation of viologen radicals in homogeneous and vesicular systems

The purpose of the present work was to study the influence of the nature of the oxidizing agent on the rate constant of the oxidation of viologen radicals in homogeneous and vesicular systems by water-soluble oxidizing agents. In this work the authors used the following water-soluble viologens: methyl viologen, sulfoethyl viologen, trimethylaminopropyl viologen, and octadecyl viologen, which is soluble only in a lipid bilayer. The rate constant for the oxidation of a viologen radical cation in a lipid bilayer by electron acceptors in the aqueous phase, in contrast to the homogeneous reaction, is lower than the rate constant of the diffusion-controlled reaction. The rate of electron transfer on the interface is determined by the interaction of the reactants with the lipid bilayer. The rate constant for electron transfer from a viologen radical cation near the inner surface of a lipid bilayer to viologen near the outer surface with the formation of a radical which is available for oxidation in the external aqueous phase has been determined and has been found to be independent of the nature of the oxidizing agent.